Numerous methods and devices for detecting particles, for example, soot or dust particles, are known from the related art.
It is believed to understood from practice to measure a concentration of particles, for example soot or dust particles, in an exhaust gas with the aid of two electrodes which are situated on a ceramic. This may take place, for example, by measuring the electrical resistance of the ceramic material separating the two electrodes from one another. Sensors of this type are used, for example, in an exhaust tract of an internal combustion engine, such as a combustion engine of the diesel design. These sensors are usually located downstream from the internal combustion engine or the diesel particulate filter. As a consequence of increasing environmental awareness and, to some extent, due to statutory regulations, soot emissions must be monitored during the driving operation of a motor vehicle and the functionality of this monitoring must be ensured in the future. This type of monitoring of the functionality is, in general, referred to as on-board diagnostics. Today, particle sensors are used, for example, for monitoring the soot emissions of internal combustion engines and for on-board diagnostics (OBD), for the purpose of monitoring the function of particle filters, for example. In this context, collecting, resistive particle sensors are known which evaluate a change in the electrical properties of an interdigital electrode structure based on particle accumulation. Soot sensors of this type are known from DE 101 49 333 A1 or WO 2003/006976 A2, for example.
In the case of this type of resistive particle sensors for conductive particles, two or several metallic electrodes are formed on an electrically insulating substrate, the particles, in particular soot particles, which accumulate under the effect of an electrical measuring voltage, short-circuiting the electrodes, which mesh in a comb-like manner, and a decreasing resistance or an increasing current being measurable at a constant, applied voltage. In order to regenerate the sensor element after the accumulation of soot, the soot is burned off the sensor element in certain phases with the aid of an integrated heating element. The evaluation of the sensor signal takes place in the system by comparing the setpoint tripping time, which is ascertained from a signal behavioral model, taking into consideration the raw emission model, and the actual sensor tripping time.
In order to monitor the functionality of the electrodes and thus that of the sensor in the field, a measuring voltage is applied to the electrodes at the end of the regeneration. This results in an ionic current which is caused most of the time by contaminants in the form of sodium. If this ionic current exceeds a certain threshold value the electrodes are to be considered to be intact.
In the case of the resistive particle sensor of the related art, the self-diagnosis of the measuring electrodes is based on a current measurement at elevated temperatures. As a result of the presence of sodium ions in the insulating layer under the electrode a certain, measurable electrical conductivity is present in this case. Therefore, this diagnosis is carried out during the sensor regeneration where an active heating is carried out anyway and temperatures >750° C. are reached.
Despite the numerous advantages of the methods and devices known from the related art for detecting particles, there is still room for improvement. For example, the type of self-diagnosis described above is resistant to aging only to a limited extent. According to the related art, the negative measuring electrode is connected to ground during this phase and the positive measuring electrode is also connected to ground except for the short diagnostic phase, which is why during operation, the positive heater terminal as well as parts of the heater always have an electrical potential which is positive thereto. Since the regeneration furthermore typically takes several seconds to minutes, positively charged particles, such as in particular the sodium ions, are subjected over this longer period of time to a driving force from the inside of the sensor, where the heater is located, to the surface, where the measuring electrode is located. As a result of the high sensor temperature during this phase, the sodium ions show a great mobility and migrate upward. This mobility may be measured as current and is referred to in the following as heater input. On the surface and in the layers close to the surface, the sodium ions are furthermore subjected to a driving force toward the negative electrode during those phases in which a positive potential is applied to the positive electrode and the sensor temperature is high. Eventually, the ions start concentrating on the surface. The heater input, meaning the movement of the sodium ions from the heater toward the surface, falsifies the measurement of the self-diagnosis current, meaning the movement of the sodium ions on the surface and into the layers close to the surface, and may thus lead to a false diagnosis result. This heater input is a function of the electrical potential applied to the heater and the proportion of the conductive ions in relation to the electrons.